[Mne_analysis] source localizing frequency bands

[Hari Bharadwaj]

Hi Kanal,
   You are right, they are are not the same. The reason they are not the
same practically is that the noise covariance when generated after
bandpass  filtering the signal first can be very very different (in
scale and possibly also in the spatial structure) and hence the inverse
operator is different.

I would vouch for computing one inverse operator with the entire band of
interest (0-100Hz for example) and then looking at narrower bands once the
data is in source space. My reason for that would be to not impose too
much temporal correlation in the noise by band-pass filtering in sensor
space. MNE as such is equivalent to a maximum aposteriori probability
(MAP) estimate of the source space activity only under the assumption that
the noise is uncorrelated in time. It is robust to violations of the
assumption to a large extent (correct me if I'm wrong here) but filtering
around 7Hz, for instance to look at alpha makes the noise heavily
different from its assumed no temporal correlation behaviour.

Having said that, if its an event related paradigm and you have enough
trials and excellent SNR, doing it either way should not make a
difference.

Regards,
Hari








On Wed, December 2, 2009 2:11 pm, Kanal Eliezer wrote:
> Just a few thoughts...
>
> I'm not familiar enough to comment on the DICS method, but the MNE method
> returns a distribution based on the provided sensor waveforms. It would
> seem intuitive to me that performing a restrictive (say, only alpha
> activity) bandpass on the sensor waveform prior to performing MNE would
> significantly change the topology of the waveform, and thus significantly
> affect how the sources are localized.
>
> The real question being asked is whether the steps are commutative; i.e.,
> do the following two analysis streams produce identical results:
> 1) bandpass, MNE
> 2) MNE, bandpass
> My gut intuition is that they are NOT, since the first method performs the
> bandpass on the raw MEG data, so to speak, while the second one performs
> the bandpass on reconstructed data. However, I can't prove this, and it
> bears further investigation. Anyone else have any thoughts?
>
> Elli Kanal
>
>
>
> On Nov 30, 2009, at 5:20 PM, Gustavo Sudre wrote:
>
>> Hello,
>>
>> I have a generic question about source localization. From what I've
>> seen so far, when the goal is to localize the activity in certain
>> frequency bands, it's common to use methods such as DICS or the
>> variant of MCE for frequency bands. I was wondering if it is also
>> correct to do source localization using a different method (e.g. MNE)
>> with the whole signal (i.e. prior to band-pass filtering to specific
>> bands) and then convert the signal in the localized sources to
>> frequency domain. Another alternative would be to band-pass the signal
>> to a certain frequency band in source space, and then run a different
>> source localization method on it (e.g. MNE).
>>
>> Could anyone elaborate on the advantages / disadvantages ( / validity)
>> of these two methods?
>>
>> Thank you,
>>
>> Gus
>>
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-- 
Hari Bharadwaj